Stages of reprogramming during mammalian development

The second lecture focused on the mechanisms involved in natural reprogramming during early development and in the germline. Studies in mice have shown that epigenetic marks such as DNA methylation and histone modification are highly dynamic. Thus, for the same genome in all or almost all cells, there are a multitude of epigenomes, each characteristic of a specific cell type and composed of epigenetic marks that may allow the memorization of gene transcriptional states across cell divisions. To take just one example, once an egg has been fertilized, it divides and some of the embryo's cells quickly migrate to the genital ridge, where they become germ cells. We now know that this differentiation is accompanied, in both male and female germ lines, by an intense reprogramming of the epigenome, characterized by the deletion of genome-wide epigenetic marks and the acquisition of new marks associated with cell totipotency/pluripotency. We have traced the dynamics of these epigenetic changes in detail, discussing the molecular mechanisms involved in each case. We also pointed out that recent breakthroughs in the field have been achieved thanks to genetic tools and the ability to explore gene expression and associated phenotypes at the single-cell level.